Geomechanical Modeling of Deformation Banding in the Navajo Sandstone, San Rafael Monocline, Utah

Deformation bands are ubiquitous geological features in many types of rocks. Depending on their micro-structure, they can act either as conduits or barriers to fluid flow. Given the significant roles deformation bands play in fluid flow and chemical transport in rocks, it is important to develop fundamental understanding of their origin, and their characteristics as they relate with the host rock properties and their depositional and structural-geological history. We present a forward-modeling technique based on the geomechanical Bifurcation Theory (BT) to predict the formation of deformation bands in sandstone. According to BT, the formation of deformation bands is a result of strain location, which in turn stems from instability in the stress-strain response of materials during loading. Due to bifurcation, a material which undergoes homogeneous deformation can reach a point at which the material experiences instability and deformation starts to become non-homogenous. We implemented BT in the commercially-available geomechanical code FLAC (Fast Langragian Analysis of Continua) and applied it in the field-scale modeling of deformation banding in the Navajo Sandstone in the San Rafael Monocline in Utah induced by fault propagation folding. The results show that geomechanical modeling using BT has a powerful potential to simulate the physical processes in the formation of deformation banding in rocks. Predicted deformation bands, specifically the pervasive bedding-parallel bands in the Navajo sandstone formation, normal faulting in the upper limb and reverse faulting in the lower limb, are generally in agreement with field observations. Predictions indicate that the pervasive bedding-parallel bands in the Navajo Sandstone are transitional compaction-shear bands with alternating zones of volumetric compaction and dilation. These predictions are consistent with petrographic analysis of thin sections of rock samples from the Navajo Sandstone. The most important parameter in the geomechanical modeling is the dilation angle in relation to the friction angle of the host rock. These parameters, as well the elastic properties, should evolve during the geologic history of a site, thus, the main challenge in the modeling is how to calibrate these parameters to yield consistent results.